In the wind industry the need for wind turbines to have light beacons to alert air traffic to their presence is well known. In fact, there are administrative regulations that require it when the maximum height that their blades reach exceeds certain pre-established limits.
In wind turbines of up to 2-3 MW, with rotors of 80-100 metres in diameter and total heights lower than 150 metres, the beacon system usually used consists in placing light sources on the nacelle of the wind turbine.
Nevertheless, in higher power wind turbines such as the new wind turbines of 4-5 MW, the rotors have diameters greater than 100 metres and the total height of the wind turbine exceeds 150 metres, which means that the beacon system should include light sources, not only on the nacelle of the generator, but also on the tip of the blade.
One known beacon system consists of placing a module with lamps or LEDs in the blade tip area, supplied with a conventional copper cable that runs from the hub of the rotor inside the blade to the tip. An example of this type of beacon system is described in WO 2006/028481.
One problem with this type of system is that it is not possible to ensure that the power supply cable to the lights or LEDs is not affected by lightning strikes despite having protection systems for this purpose. Moreover, there is the risk that this power supply cable acts as a point of entry for the lightning into the hub of the rotor and other areas of the wind turbine.
To avoid damage due to lightning strikes, both the wind turbine in general and the blades in particular are equipped with lightning protection systems. The wind turbine blades, typically manufactured from composite materials, constitute the most likely point for direct lightning strikes. For this reason, the majority of wind turbine blades are equipped with lightning receptors on the tip and in other areas of the blade connected to a lightning conductor cable that runs the entire length of the blade from the tip to the root. In the case of lightning striking the blade, the electromagnetic field induced in the area surrounding the conductor cable endangers any additional conductive elements in the blade. The lightning protection standards state that any conductive element in the blade must be connected electrically to the main lightning protection system of the blade. Nevertheless, this connection must be made carefully avoiding the deterioration of the conductive elements. In cases where the additional conductive elements are confined to a small and clearly delimited space, the protection system may consist of including these elements in a Faraday cage and connecting them through the protection devices belonging to the main lightning conductor cable. Nevertheless, if the additional conductive elements are long and are parallel to the main earthing conductor, their protection system is more complicated and its efficacy is not guaranteed. In the case of having a cable that runs parallel to the main lightning conductor cable, no system can be guaranteed to protect this cable against the effects of lightning with 100% efficacy.
Another known beacon system consists in placing the light sources on the hub of the rotor and guiding the light up to the tip of the blade using fibre optics or other means of transmission.
The documents WO 03/050412 and WO 2007/068251 describe systems like this.
These systems have the advantage that they do not include conductive elements in the blade and therefore are not exposed to the effects of lightning, but they have the inconvenience of a low efficiency in light transmission. When using fibre optics as means of transmission, the optimum efficiency that can be achieved when coupling the light to the fibre optic is very low (no more than 10%) and the resulting system is energy inefficient and costly. In addition, with current fibre optic technology and for the wavelength that must be used for lighting the tip of the blade (˜620 nm) the power losses caused in a blade longer than 50 metres are very high.
This invention is directed at the solution of this problem.